ESTRO 36 Abstract Book

S83 ESTRO 36 2017 _______________________________________________________________________________________________

Purpose or Objective Kilovoltage cone-beam computed tomography (kVCBCT) has often been regarded as the preferred imaging modality for the visualisation of soft tissues and verification of treatment position due to its superior spatial resolution [1-3]. Transperineal ultrasound (TPUS) is an alternative imaging tool that can be employed for pre-treatment verification and in-treatment monitoring as it is non- invasive and does not involve additional imaging dose [4, 5]. This study aimed to compare the daily inter-modality derived setup shifts using TPUS versus kVCBCT (gold standard) for prostate radiotherapy. Material and Methods A total of 1927 paired datasets (TPUS versus kVCBCT) from 55 patients were compared in three directions (i.e. x,y,z shifts representing left/right, anterior/posterior and superior/inferior directions respectively). The derived setup shifts were reported to the nearest mm. Data were analysed using PASW for windows, version 20.0 (SPSS Inc, Chicago, IL). Observed differences in the derived shifts for each imaging modality were reported. Statistical tests were conducted under a two-tailed significance level, at a minimum 95% confidence interval. Results A Shapiro-Wilk test revealed that the data was not normally distributed (p<0.05). A non-parametric Wilcoxon Signed Ranks test demonstrated no statistically significant difference between the derived setup shifts from TPUS and kVCBCT for all planes; x (p= 0.376), y (p=0.244) and z (p=0.253). The proportion (%) of datasets where the difference in the derived shifts between the two imaging modalities were within 5/4/3mm in the x, y and z directions are reported in Table 1. Spearman’s rank correlation coefficients of the derived shifts were moderate (0.612-0.671) for all three directions (p<0.005), signifying that the accuracy of TPUS-derived setup shifts was comparable to kVCBCT. Table 1: Proportion of datasets where the difference in the derived shifts between the two imaging modalities were within 5/4/3mm in the x, y and z directions.

the tabletop for the patients’ positioning devices has to provide a reproducible position. The treatment plan will be adapted to the patients’ position, anatomical variation and organ motion of the day. The aim of this study was to characterize the accuracy of patient set-up with a table indexing system in the absence of lasers and skin marks. Material and Methods This investigation was performed on a conventional MRI. MR-scans were acquired at 3 different time points from 8 volunteers. The pelvis was chosen as anatomical region of interest. The tabletop of this MRI is comparable with the tabletop of the MR-Linac. A head support and a knee support, indexed on the table, were used for stability and reproducibility. The first MR-scan was defined as reference scan and the following two MR-scans were registered to this reference on bony anatomy. The setup variability was analyzed in terms of group mean (M), systematic (Σ) and random errors (σ) for both translations and rotations. The results were compared to retrospective set-up data of 79 patients treated for rectum carcinoma (5x5 Gy), aligned with lasers and skin marks and measured with CT-scan and Cone Beam-CT position verification. Because the group of volunteers is relatively small, the comparison to the rectal cancer patient group is on a descriptive basis only. Results When comparing retrospective set-up data of rectal cancer patients to the group of volunteers in this investigation, for translations, the group mean for the patient group seem to show a better set-up reproducibility in the LR and CC direction as compared to the volunteers. This resulted in group means closer to zero with corresponding smaller Σ errors and σ errors. In the AP direction, the mean and standard errors did not seem to show apparent differences. For rotations the results for both groups were comparable. The results are presented in table 1.

Conclusion Measured differences were acceptable considering the planning target volume (PTV) margin expansion was 10mm in all directions, except posteriorly (6mm). Findings were in agreement with the recent report by Trivali et al. [6] who found no significance difference in the x, y and z coordinates between TPUS and fiducial-based CT localisation of the prostate gland. With specialised training and user experience, TPUS is a promising imaging modality in treatment setup and verification for prostate radiotherapy without the need for additional exposure to ionising radiation.

Conclusion For volunteers, without the use of laser alignment, translations seem to be larger in LR and CC direction. Rotations were comparable for both groups. However, for daily practice, the impact of this increased uncertainty is likely small relative to uncertainties of internal organ motion that can be in the cm range. In daily on-line corrections, the combination needs to be considered in positioning pelvic cancer patients without skin marks, on an MR-Linac. OC-0165 TPUS vs CBCT: comparison of daily inter- modality derived setup shifts for prostate radiotherapy. E.P.P. Pang 1,2 , K. Knight 2 , M. Baird 2 , J.M.Q. Loh 1 , E.T.Y. Chen 1 , G.K. Low 1 , C.C.C. Yap 1 , A.H.S. Boo 1 , J.K.L. Tuan 1,3 1 National Cancer Centre Singapore, Division of Radiation Oncology, Singapore, Singapore 2 Monash University, Faculty of Medicine- Nursing and Health Sciences Medical Imaging & Radiation Sciences, Melbourne, Australia 3 Duke-NUS, Graduate Medical School, Singapore, Singapore